He fought far more at both heavyweight and middleweight. Heavyweight is probably what he's best known for fighting at, and he was a natural middleweight, surely either of those would make more sense for him to be ranked as.
Since I don't think it's worth it's own thread a similar case is Jem Mace, who never even fought when L.Heavyweight existed, again fighting at middleweight and heavyweight, and given he was arguably a heavyweight world champion, I'd probably suggest that as his listed weight division.
http://boxrec.com/en/boxer/40325